The present invention relates to a current mirror circuit and, more particularly, to a current mirror circuit that is suited for use with a low supply voltage and is capable of maintaining a current ratio accuracy over a wide range of mirror ratios and input currents.
Current mirrors are often used in electronic circuits to provide an output current that is related to an input current. The output current may be identical to the input current or may be a fixed ratio of the input current as is the case of a current gain or current reduction block. In addition, the input current to the current mirror may be fixed and remain constant or may be variable. An important characteristic of a current mirror for good dynamic performance is to maintain a linear relationship between the input current and the output current over a wide range of input currents and mirror ratios.
A characteristic that is especially important for current mirrors that are for use with low supply voltages is the voltage compliance for each of the input and output terminals. The voltage compliance of the input and output terminals is the range of voltages at each of the input and output terminals for which the current mirror provides an accurate mirror ratio. To maximize the voltage compliance of the input and output terminals of the current mirror, the voltage drop between a supply terminal and each of the input terminal and the output terminal should be minimized. For example, the supply voltage for the current mirror must be greater than or equal to the sum of the input voltage across an external input current source and the voltage between the input terminal and the supply terminal. The external input current source is connected between a reference terminal, or ground terminal, and the input terminal. Therefore, for low supply voltage operation, it is necessary that the voltage drop between the input terminal and the supply terminal be minimized so that either maximum voltage compliance at the input terminal or maximum "headroom" can be provided. The headroom is the difference between the supply voltage and the sum of the input voltage across the external input current source and the voltage between the input terminal and the supply terminal. In a similar manner, it is necessary that the voltage drop between the output terminal and the supply terminal be minimized to provide maximum voltage compliance at the output terminal or to provide maximum headroom between the output terminal and a reference terminal or ground terminal.
There is a present need for a current mirror circuit that provides a relatively low voltage drop between the supply terminal and both input and output terminals of the current mirror thereby allowing the supply voltage to be reduced without reducing either the input voltage or the headroom. In addition, this current mirror should provide a mirror ratio that is accurate over a wide range of mirror ratios. Furthermore, the current mirror should maintain linearity over a wide range of input currents.